Blood alcohol content, or BAC, is expressed as a percent and is defined as grams of alcohol per 100 milliliters of blood. A person's blood alcohol content can be determined by measuring the alcohol content of his breath. The assumption is then made that the ratio by mass of the alcohol content of the blood to that of the breath is 2100:1.
There are several methods that use a person's breath to determine his BAC. A common method is to use a tin-oxide semiconductor alcohol sensor that is exposed to a person's breath. It has the advantage of low cost at the expense of accuracy, alcohol specificity, and electrical power consumption. Another method is to employ the use of an electrochemical fuel cell alcohol sensor. While this type of sensor tends to be more accurate, more alcohol specific, and utilizes less electrical power, the sensor itself is significantly more expensive and has traditionally required the use of an active sampling mechanism, such as a pump, that samples a predetermined volume of breath. For example, Gammenthaler (U.S. Pat. No. 6,026,674) discloses an apparatus for determining the alcohol concentration in a gaseous mixture. The apparatus utilizes a fuel cell and a valve. The valve diverts a portion of the breath flow into the fuel cell thereby indicating and ensuring that a predefined amount of breath flow has passed through the fuel cell. The predetermined volume is calculated by integrating breath flow over time with the valve open and then closing the valve when the predetermined limit is reached. An electrochemical sensor responds differently to varying volumes of an alcohol gas sample. Since the traditional sampling mechanism samples a predetermined and constant volume of breath, the method for calculating the alcohol content of the breath does not need to take into account the total exhaled volume of breath, as does an apparatus without a sampling mechanism that allows for varying volumes of breath.
Chang et al. (U.S. Pat. No. 3,966,579) disclose an apparatus for measuring alcohol concentrations utilizing an electrochemical fuel cell alcohol sensor without an active sampling mechanism. Chang et al. monitor alcohol concentrations present in a gaseous breath by measuring the magnitude of the short circuit passing through the external circuit between the anode and cathode of the fuel cell. However, Chang et al. fail to disclose a method for detecting and calculating gaseous component levels of the breath which accounts for volume of the breath received.
In addition, it is desirable to discriminate components different from ethanol in breath samples. These contaminants can lead to error conditions such as faulty readings. For example, it is known that cigarette or cigar smoke can cause fuel cell gas sensors to report inaccurate gas component levels. Other error conditions could be elevated readings due to other volatile components in the breath. Chow (U.S. Pat. No. 5,048,321) discloses a method of discriminating alcohols different from ethanol in breath samples.
Accordingly, it is desirable to have a breath detection method and apparatus that utilizes an electrochemical fuel cell alcohol sensor for accuracy, alcohol specificity, and low power consumption, and eliminates the need for a sampling mechanism, saving more in cost, power consumption, and size. However, eliminating the sampling mechanism requires an improved method of calculating the alcohol content of the breath that takes into account the total exhaled volume of breath. In addition, since an electrochemical sensor in an apparatus without a sampling mechanism can respond to gases other than alcohol that are typically found in expired cigarette, cigar, or pipe smoke and cause an error condition in the fuel cell, a method of detecting such an error condition is also desired.